A SOFC is an electrochemical device for the generation of electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based). The device is generally ceramic-based, using an oxygen-ion conducting metal-oxide derived ceramic as its electrolyte. As most ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) only demonstrate technologically relevant ion conductivities at temperatures in excess of 500° C. (for cerium-oxide based electrolytes) or 650° C. (for zirconium oxide based ceramics), SOFCs operate at elevated temperatures.
In common with other fuel cells, SOFCs include an anode where fuel is oxidised, and a cathode where oxygen is reduced. These electrodes must be capable of catalysing the electrochemical reactions, be stable in their respective atmospheres at the temperature of operation (reducing on the anode side, oxidising on the cathode side), and be able to conduct electrons so the electric current generated by the electrochemical reactions can be drawn away from the electrode-electrolyte interface.
Various materials have been explored for use as cathodes in SOFCs including perovskite cobalt crystals. Barium and lanthanide containing materials such as BSCF and LSCF (barium/lanthanum, strontium and iron containing cobalt oxides) are examples of such materials and perform well as SOFC cathodes due to their high oxygen ion conductivity and area specific resistance (ASR).
However, many such materials (such as conventional (undoped) BSCF) suffer significantly from poor thermal and chemical stability. BSCF in particular reacts with various electrolyte materials while sintering (at ≥900° C. with cerium oxide based electrolytes, the most common electrolyte type with BSCF in terms of SOFC operating temperatures) and undergoes a phase transition from cubic to hexagonal polymorph at ≤900° C. (which is the typical operating temperature for the material) detrimental to its transport and catalytic properties and so increasing ASR over time, thus eliminating it from the practical use in SOFC applications.
Therefore, it is desirable to develop materials which have a comparable or lower ASR to BSCF and LSCF in low and intermediate temperature applications; yet which are more stable and, in particular, which exhibit reduced phase transition and hence have the ability to maintain lower ASR over time.
Some work has been done to augment the properties of these materials in order to improve oxygen ion conductivity, increase thermal stability and enhance resistance to degradation. For instance, heavy doping of BSCF with molybdenum has been found to improve conductivity and also improve the stability of the material whilst keeping the ASR values comparable to that of BSCF.
Unfortunately, many doped materials when used in SOFCs suffer a “leeching” phenomenon where the dopant comes out of the cathode material (e.g. to form (Ba/Sr)MoO4) and the performance of the cathode diminishes. Further, if too much of the dopant is allowed to leech out of the cathode material, then structural rearrangements can occur within crystal structures which can cause the electrode materials to fracture and decrease performance
Demont, A., et al., “Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide”, J. Am. Chem. Soc., 2013, 135, p. 10114-10123 discloses the use of molybdenum as a dopant material for making perovskite structures.
It is therefore also desirable to develop perovskite crystals and ceramics which not only demonstrate improved performance over conventional materials but that also resist leeching of dopants under SOFC operating conditions.
The invention is intended to solve or at least ameliorate some of the problems outlined above.